During the last decade vast sums have been spent in an attempt to improve the gas mileage in modern high-compression automobile engines. Attempts have been made to operate the engines with lean fuel-air mixtures and lead-free gasoline, but it has been difficult to obtain satisfactory performance in this way because of ignition or combustion problems when the air-fuel ratio is high, particularly when it exceeds 17 to 1. It is also difficult to avoid knocking and to achieve good all around performance in lean-burn engines using lead-free or low octane gasoline and compression ratios such as 9:1 to 10:1 or higher.
Ignition problems are less severe in a lean-burn engine when torch ignition is employed as in Honda-type engines which have the spark plug located in an auxiliary combustion chamber. However, the combustion problem is still a limiting factor and the engines cannot operate satisfactorily with an air-fuel ratio of 20 to 1. The large amounts of nitrogen apparently interfere with proper combustion.